1. Field of the Invention
The present invention relates to image sensing devices and image sensing systems.
2. Description of the Related Art
An image sensing device comprises a pixel array in which a plurality of pixels are arrayed in a direction along rows and a direction along columns. A line noise which extends in a horizontal direction (hereinafter referred to as “horizontal line noise”) sometimes occurs in images captured by image sensing devices.
Specifically, horizontal line noise sometimes occurs due to a generation mechanism shown in Japanese Patent Laid-Open No. 2006-128704. As shown in FIG. 2 of Japanese Patent Laid-Open No. 2006-128704, parasitic overlap capacitances CP(0) to CP(m) are formed between gates of load transistors ML(0) to ML(m) and column analog buses Bus(0) to Bus(m). When a data voltage is read out after a reset voltage has been read out from the pixels to the column analog buses Bus(0) to Bus(m) to carry out CDS processing, the voltages of the column analog buses Bus(0) to Bus(m) change from the reset voltage to the data voltage. This change in voltage affects the bias voltages via the overlap capacitances CP(0) to CP(m). The bias voltages are transmitted from an active transistor MF to the load transistors ML(0) to ML(m). In this way, the gate voltages of the load transistors ML(0) to ML(m) fluctuate, and therefore the electric currents that flow in the column analog buses Bus(0) to Bus(m) also fluctuate. Due to this fluctuation in bias voltages, proper values are not achieved for image signals, which are a difference between the reset voltages and the data voltages when capturing a bright subject, and this is a cause of horizontal line noise. That is, the more drastic the fluctuations in the voltages of the column analog buses become, that is, the brighter the brightness of a subject become, the more drastic the fluctuations in bias voltages become so that horizontal line noise increases.
In respect to these issues, Japanese Patent Laid-Open No. 2006-128704 proposes increasing the driving force of the active transistors MF by increasing their size and electric current, and stabilizing the bias voltages. In this way, horizontal line noise can be decreased according to Japanese Patent Laid-Open No. 2006-128704.
It should be noted that in the technique shown in Japanese Patent Laid-Open No. 2006-128704, the sizes of the load transistors ML(0) to ML(m) are kept the same as conventional sizes. According to Japanese Patent Laid-Open No. 2006-128704, this is to suppress decrease in the dynamic range of the pixels due to increased electric currents of the analog buses.
However, horizontal line noise is sometimes not decreased even when using the technique shown in Japanese Patent Laid-Open No. 2006-128704. For example, even when using the technique shown in Japanese Patent Laid-Open No. 2006-128704, horizontal line noise is still sometimes conspicuous in images obtained by capturing an object at dark times or by capturing extremely dark objects.
Specifically, horizontal line noise sometimes occurs due to a separate generation mechanism that is not described in Japanese Patent Laid-Open No. 2006-128704. As shown in FIG. 2 of Japanese Patent Laid-Open No. 2006-128704, when an interface state is present in a gate insulating film of the active transistors MF, that interface state captures and releases electrons and positive holes that form an electric current Ibias flowing in a channel, and therefore fluctuation (flicker noise) occurs in that electric current Ibias. This fluctuation in the electric current Ibias affects bias voltages that are transmitted from the active transistors MF to the load transistors ML(0) to ML(m). In this way, the gate voltages of the load transistors ML(0) to ML(m) fluctuate, and therefore electric currents Ibias*x that flow to the column analog buses Bus(0) to Bus(m) also fluctuate. In response to this, the values of the reset voltages and data voltages that are read out from the pixels to the column analog buses Bus(0) to Bus(m) fluctuate undesirably. Due to this fluctuation in bias voltages, proper values (substantially zero) are not achieved for image signals, which are a difference between the reset voltages and the data voltages when a subject is captured in a state close to no signal, that is, when capturing an object at a dark time or capturing an extremely dark object, and this is a cause of horizontal line noise.